Radio emission from a nearby M dwarf binary

Abstract

We present the detection of the binary system 2MASS J02132062 + 3648506 AB using the Karl G. Jansky Very Large Array archive data observed at 4–8 GHz. The system is a triple consisting of a tight binary (∼0 . 2arcsec ) of two M dwarfs of spectral types M4.5 and M6.5 and a wide T3 brown dwarf companion ( ∼16.4 arcsec). The binary displays coronal and chromospheric activity as traced by previously measured X-ray flux and H αemission. We detect the unresolved binary at a peak flux density of∼356μJybeam −1 at a signal-to-noise ratio (SNR) of∼36 and determine a radio luminosity oflog LR /log Lbol ∼−7 . 76. The radio emission is quiescent, polarized at a mean circular polarization fraction fc = 45 . 20 ±1 . 58% and exhibits a spectral index α= −0 . 44 ±0 . 07. We probe the binary using the Enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN) with an angular resolution of∼40 mas at 5 GHz and detect a component at a peak flux density of∼90μJybeam −1 at a SNR ∼5. We propose a gyrosynchrotron origin for the radio emission and estimate a magnetic field strength B < 174 . 86 G, an emitting region of size L < 1 . 54 times the radius of the M4.5 primary and a plasma number density ne < 2 . 91 ×105 cm −3 . The brown dwarf companion is not detected. Additionally, we have analysed observations of 2MASS J04183483 + 213127, a chromospherically active L5 brown dwarf which is also not detected and can only place 3 σflux density upper limits at 36 . 9μJybeam −1 and 42 . 3μJybeam −1 for Stokes I and V, respectively.